Which is More Acidic: Ethane or Ethene?

Introduction

Both molecules are hydrocarbons with C-H bonds. Neither is a strong acid, but one is measurably more acidic than the other. The key is in how the carbon is hybridized.

Acidic Protons

The acidic protons are the C-H hydrogens. In ethane, C is sp3-hybridized. In ethene, C is sp2-hybridized. The hybridization of the carbon holding the proton determines how tightly it holds the electrons.

Governing Factor: Hybridization

The governing factor is hybridization. sp2 orbitals have 33% s-character vs 25% for sp3. s orbitals are closer to the nucleus, so sp2 carbon holds bonding electrons more tightly - making it more electronegative and better at stabilizing a carbanion.

Conjugate Base Stability

When ethane loses H+, the carbanion sits in an sp3 orbital (25% s-character). When ethene loses H+, the carbanion sits in an sp2 orbital (33% s-character). The sp2 carbanion is more stable because the electrons are held closer to the nucleus.

pKa Comparison

Ethane has a pKa of ~50 and ethene has a pKa of ~44. Both are very weak acids, but the 6-unit difference shows that hybridization has a real effect even among C-H bonds.

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